To meet the recent demand for higher integration in integrated circuits, pattern formation to a finer feature size is required. In forming resist patterns with a feature size of 0.2 μm or less, chemically amplified resist compositions utilizing photo-generated acid as the catalyst are typically used in the art because of their high sensitivity and resolution. Often, high-energy radiation such as UV, deep UV or electron beam (EB) is used as the light source for exposure of these resist compositions. Among others, the EB or EUV lithography is recognized most attractive because patterns of the finest size are expectable.
Resist compositions include positive ones in which exposed areas become soluble and negative ones in which exposed areas are left as a pattern. A suitable composition is selected among them depending on the desired resist pattern. In general, the chemically amplified negative resist composition comprises a polymer which is normally soluble in an aqueous alkaline developer, an acid generator which is decomposed to generate an acid when exposed to light, and a crosslinker which causes the polymer to crosslink in the presence of the acid serving as a catalyst, thus rendering the polymer insoluble in the developer (sometimes, the crosslinker is incorporated in the polymer). Typically a basic compound is added for controlling the diffusion of the acid generated upon light exposure.
A number of negative resist compositions of the type comprising a polymer which is soluble in an aqueous alkaline developer and includes phenolic units as the alkali-soluble units were developed, especially as adapted for exposure to KrF excimer laser light. These compositions have not been used in the ArF excimer laser lithography because the phenolic units are not transmissive to exposure light having a wavelength of 150 to 220 nm. Recently, these compositions are recognized attractive again as the negative resist composition for the EB and EUV lithography capable of forming finer size patterns. Exemplary compositions are described in JP-A 2006-201532 (corresponding to US 20060166133, EP 1684118, CN 1825206) and JP-A 2006-215180.
As the required pattern size is reduced, more improvements are made on the negative resist composition of the type using hydroxystyrene units typical of the phenolic units. Now that the pattern reaches a very fine size of 0.1 or less, there is a likelihood that the resist layer is left like thin strings between features of a fine size pattern, which is known as “bridge problem.” The prior art compositions fail to solve the problem.
Also known in the art is the problem of pattern's substrate dependence, that is, the profile of a pattern alters near a processable substrate, depending on the material of which the substrate is made. As the desired pattern size is reduced, even a minor profile alteration becomes significant. Particularly in processing a photomask blank having the outermost surface made of chromium oxynitride, if a pattern is formed on the chromium oxynitride using a chemically amplified negative resist composition, then an “undercut” problem arises, that is, the pattern is notched at positions in contact with the substrate. The prior art compositions fail to fully solve the undercut problem.